Ultrasonic instruments are often used in surgery to cut and coagulate tissue. Exciting the end effector (e.g. cutting blades) of such instruments at ultrasonic frequencies induces longitudinal vibratory movement which generates localized heat within adjacent tissue, facilitating both cutting and coagulation. Because of the nature of ultrasonic instruments, a particular ultrasonically actuated end effector may be designed to perform numerous functions, including, for example, cutting and coagulation. The structural stress induced in such end effectors by vibrating the blade at ultrasonic frequencies may have a number of undesirable effects. Such undesirable effects may include, for example, substantial transverse motion in the instrument waveguide which may lead to, for example, excess heat generation in the waveguide or premature stress failure. The undesirable effects of vibrating a surgical end effector at ultrasonic frequencies are compounded where the end effector is not symmetrical, that is, where the mass of the end effector is not distributed symmetrically about a line extending through the central axis of the transmission waveguide. Therefore, one way to improve the performance of ultrasonically actuated end effectors is to design end effectors which are substantially symmetric about the central axis of the transmission waveguide. Alternatively, the surgical end effector may be small and short, in which case the end effector will act like a small lumped mass at the end of the transmission waveguide and will not induce substantial transverse motion in the waveguide. Where it is desirable to design end effectors which are not symmetric, performance may be improved by designing the end effector such that the center of mass of the end effector is located along a line which extends through the central axis of the waveguide. One known method of moving the center of mass is to add or subtract mass opposite or close to the asymmetric region until the center of mass lies along a central axis. As a further alternative, longitudinal vibratory motion in the waveguide may be reduced or eliminated by using thicker, more robust waveguides which are not as subject to transverse vibratory motion. However, the use of thick waveguides may not be an acceptable alternative where the ultrasonic surgical instrument is being designed for use in minimally invasive surgery such as endoscopic or laparoscopic surgery. In such instruments it is generally desirable to reduce the diameter of the ultrasonic waveguide in order to fit the instrument through the tiny surgical incisions, narrow body orifices and through trocars presently being used and being designed for future procedures. Long thin ultrasonic waveguides, such as those used in instruments for minimally invasive surgery, are particularly susceptible to transverse vibrations introduced by imbalances in the end effector.
For certain applications, it is desirable to include one or more axially asymmetrical features, (e.g. blade curvature) to enhance performance of the end effector. It may also be desirable to design such end effectors to be relatively long, in order to facilitate certain surgical procedures. In such end effectors, it is not always possible or desirable to include opposed balancing features in the treatment portion in order to balance the end effector by aligning the center of mass with the central axis of the transmission waveguide. It would, therefore, be desirable to develop a method of designing an ultrasonic surgical instrument including a waveguide and an ultrasonic end effector wherein undesirable transverse vibrations resulting from the inclusion of beneficial asymmetrical features (e.g. a long curved blade) in the working portion of the end effector have been reduced or eliminated. It would further be advantageous to develop a method of designing such an instrument wherein the undesirable transverse vibrations have been reduced or eliminated without adding balancing features to the treatment portion of the end effector. It would further be advantageous to develop a method of designing an end effector wherein undesirable transverse vibrations resulting from the inclusion of beneficial asymmetrical features in the treatment portion of the end effector have been reduced or eliminated by adding asymmetrical balancing features proximal to the treatment portion of the end effector. It would further be advantageous to develop a method of designing an asymmetric end effector with a center of mass which is not on the central axis of the transmission wave guide wherein significant transverse motion is not induced in the waveguide by the asymmetric end effector.